How to white out screen with a flare
This can be done very easily in the compositor. I am using the Cycles node set in this example.

First, press Shift+A and add an Ellipse Mask node.



Do the same for a Blur node.



Then hook up the nodes as shown. Animate the Width and Height values by keyframes to animate the effect.



Depending on your render size you may need to adjust the blur size, but this should get you on the right track.